U.S. patent application number 11/970181 was filed with the patent office on 2008-09-04 for cascaded optical amplifier and control method thereof.
This patent application is currently assigned to BOOKHAM TECHNOLOGY PLC. Invention is credited to Selina G. FARWELL, Kevan JONES, Andre VAN SCHYNDEL.
Application Number | 20080212167 11/970181 |
Document ID | / |
Family ID | 39361377 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080212167 |
Kind Code |
A1 |
VAN SCHYNDEL; Andre ; et
al. |
September 4, 2008 |
CASCADED OPTICAL AMPLIFIER AND CONTROL METHOD THEREOF
Abstract
A cascaded optical amplifier including a first optical amplifier
and a second optical amplifier in cascaded arrangement is provided.
Each of the first optical amplifier and the second optical
amplifier has a respective input for receiving an optical signal,
an output for outputting an amplified optical signal, and a control
input for controlling the gain of the optical amplifier. The
cascaded optical amplifier includes a sensor for sensing upstream
of the input of the second optical amplier a signal relating to
operation of the cascaded optical amplifier. In addition, the
cascaded optical amplifier includes a controller for providing
control signals to the respective control inputs of the first
amplifier and the second amplifier, the controller providing the
control signal to the second optical amplifier as a function of the
sensed signal.
Inventors: |
VAN SCHYNDEL; Andre;
(Kanata, CA) ; JONES; Kevan; (Kanata, CA) ;
FARWELL; Selina G.; (Paignton, GB) |
Correspondence
Address: |
MARK D. SARALINO (GENERAL);RENNER, OTTO, BOISSELLE & SKLAR, LLP
1621 EUCLID AVENUE, NINETEENTH FLOOR
CLEVELAND
OH
44115-2191
US
|
Assignee: |
BOOKHAM TECHNOLOGY PLC
Northamptonshire
GB
|
Family ID: |
39361377 |
Appl. No.: |
11/970181 |
Filed: |
January 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60883869 |
Jan 8, 2007 |
|
|
|
Current U.S.
Class: |
359/341.41 ;
359/341.5 |
Current CPC
Class: |
H04B 10/2935 20130101;
H01S 3/06758 20130101 |
Class at
Publication: |
359/341.41 ;
359/341.5 |
International
Class: |
H01S 3/00 20060101
H01S003/00 |
Claims
1. A cascaded optical amplifier, comprising: a first optical
amplifier and a second optical amplifier in cascaded arrangement,
each of the first optical amplifier and the second optical
amplifier having a respective input for receiving an optical
signal, an output for outputting an amplified optical signal, and a
control input for controlling the gain of the optical amplifier; a
sensor for sensing upstream of the input of the second optical
amplifier a signal relating to operation of the cascaded optical
amplifier; and a controller for providing control signals to the
respective control inputs of the first optical amplifier and the
second optical amplifier, the controller providing the control
signal to the second optical amplifier as a function of the sensed
signal.
2. The amplifier of claim 1, wherein the sensed signal represents
the optical signal input to the first optical amplifier.
3. The amplifier of claim 1, wherein the sensed signal represents
the amplified optical signal output from the first optical
amplifier.
4. The amplifier of claim 1, wherein the sensor comprises first and
second sensors, and the sensed signal comprises a first sensed
signal representing the optical signal input to the first optical
amplifier and a second sensed signal representing the amplified
optical signal output from the first optical amplifier.
5. The amplifier of claim 1, further comprising at least a third
optical amplifier included in the cascaded arrangement between the
first and second optical amplifiers.
6. The amplifier of claim 1, wherein the controller provides the
control signal to the second optical amplifier based on a
comparison of the amplified optical signal output or the pump drive
signal from the second optical amplifier and the sensed signal.
7. The amplifier of claim 6, wherein the comparison comprises a
ratio.
8. The amplifier of claim 1, wherein the controller includes a
sensed signal delay element for providing a delay to the sensed
signal, the amount of the delay being determined to synchronize
approximately the sensed signal received by the controller with at
least one other signal received by the controller for carrying out
control.
9. The amplifier of claim 1, wherein the first optical amplifier
and the second optical amplifier are erbium doped fiber
amplifiers.
10. The amplifier of claim 1, further comprising a delay
introducing element coupled between the output of the first optical
amplifier and the input of the second optical amplifier, and the
sensed signal is sensed upstream of the delay introducing
element.
11. The amplifier of claim 10, wherein the controller delays the
sensed signal as a function of the delay introduced by the delay
introducing element as measured by the controller.
12. The amplifier of claim 10, wherein the delay introducing
element is a dispersion compensation module (DCM).
13. A method of controlling a cascaded optical amplifier, the
cascaded optical amplifier comprising: a first optical amplifier
and a second optical amplifier in cascaded arrangement, each of the
first optical amplifier and the second optical amplifier having a
respective input for receiving an optical signal, an output for
outputting an amplified optical signal, and a control input for
controlling the gain of the optical amplifier; and a controller for
providing control signals to the respective control inputs of the
first optical amplifier and the second optical amplifier, the
method comprising the steps of: sensing upstream of the input of
the second optical amplifier a signal relating to operation of the
cascaded optical amplifier; and providing the control signal to the
second optical amplifier as a function of the sensed signal.
14. The method of claim 13, wherein the sensed signal represents
the optical signal input to the first optical amplifier.
15. The method of claim 13, wherein the sensed signal represents
the amplified optical signal output from the first optical
amplifier.
16. The method of claim 13, wherein the sensing step comprises
sensing a first sensed signal representing the optical signal input
to the first optical amplifier and sensing a second sensed signal
representing the amplified optical signal output from the first
optical amplifier.
17. The method of claim 13, comprising the step of providing the
control signal to the second optical amplifier based on a
comparison of the amplified optical signal output or the pump drive
signal from the second optical amplifier and the sensed signal.
18. The method of claim 17, wherein the comparison comprises a
ratio.
19. The method of claim 13, comprising the step of providing a
delay to the sensed signal, the amount of the delay being
determined to synchronize approximately the sensed signal received
by the controller with at least one other signal received by the
controller for carrying out control.
20. The method of claim 13, wherein the first optical amplifier and
the second optical amplifier are erbium doped fiber amplifiers.
21. The method of claim 13, wherein the cascaded amplifier further
comprises a delay introducing element coupled between the output of
the first optical amplifier and the input of the second optical
amplifier, and the sensed signal is sensed upstream of the delay
introducing element.
22. The method of claim 21, comprising the step of delaying the
sensed signal as a function of the delay introduced by the delay
introducing element as measured in a measuring step.
23. The method of claim 21, wherein the delay introducing element
is a dispersion compensation module (DCM).
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 U.S.C. .sctn.119
to U.S. Provisional Application No. 60/883,869, filed Jan. 8, 2007,
the entire disclosure of which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates generally to optical
amplifiers, and more particularly to two or more optical amplifiers
in cascaded arrangement.
BACKGROUND OF THE INVENTION
[0003] Erbium doped fiber amplifiers (EDFAs) are used extensively
alone or in subsystems to amplify fiber optic signals in single
channel and dense wavelength division multiplexing (DWDM) optical
networks. The EDFA has the capability of passing energy from a
"pump" laser to the optical signal to be amplified. The gain of the
EDFA is a function of the input, the pump power and their
corresponding history (e.g., over the previous milliseconds).
[0004] Subsystems and module products available in recent years
contain two or more EDFAs cascaded together and separated by a
dispersion compensation module (DCM) which can by the nature of its
design also introduce a delay. For example, FIG. 1 shows a
conventional two-stage cascaded optical amplifier 14. An add/drop
type optical signal is input to a first EDFAa and its amplified
output is input to a second EDFAb. The output of EDFAa is coupled
to the input of EDFAb via a DCM with its corresponding delay. The
output of EDFAb represents the output of the cascaded
amplifier.
[0005] A first control algorithm 20 provides gain control of EDFAa.
A power coupler or tap 22 senses the power of the optical signal
input to EDFAa and provides a control input PINa1 to the control
algorithm 20. Similarly, a tap 24 senses the power of the amplified
optical signal output by EDFAa and provides a control input PINa2
to the control algorithm. The control algorithm 20 compares the
output power of EDFAa to the input power of EDFAa. Based on the
desired gain of EDFAa, the control algorithm 20 provides a gain
control signal to EDFAa in the form of a pump control signal to
Pump a. By controlling the laser pump energy delivered by Pump a,
the control algorithm 20 controls the gain provided by EDFAa.
[0006] The amplified optical signal output from EDFAa is coupled to
the input of EDFAb via a DCM 26. EDFAb in turn further amplifies
the optical signal output from EDFAa. A second control algorithm 30
serves to provide gain control of EDFAb. Specifically, a tap 32
outputs a control input PINb1 indicative of the power of the input
signal to EDFAb, and a tap 34 provides a control input PINb2
indicative of the power of the optical signal output by EDFAb. The
control algorithm 30 receives the control inputs PINb1 and PINb2
and based thereon compares the input and output signal power of
EDFAb with the desired gain. Based on such comparison, the control
algorithm 30 controls the laser pump energy delivered by Pump b,
which in turn controls the gain of EDFAb.
[0007] Cascaded optical amplifiers such as that shown in FIG. 1
have generally provided satisfactory results. However, there have
been certain drawbacks or disadvantages that have led to less than
optimum performance. For example, errors or noise introduced by
amplifiers upstream tend to accumulate and are exaggerated by
amplifiers downstream in the cascade.
[0008] In view of the aforementioned shortcomings associated with
existing cascaded optical amplifiers, there is a strong need in the
art for a cascaded amplifier that is less prone to the accumulation
of errors and/or noise. Moreover, there is a strong need in the art
for a cascaded amplifier in which downstream amplifiers exhibit an
improved dynamic response.
SUMMARY OF THE INVENTION
[0009] According to one aspect of the invention, a cascaded optical
amplifier including a first optical amplifier and a second optical
amplifier in cascaded arrangement is provided. Each of the first
optical amplifier and the second optical amplifier has a respective
input for receiving an optical signal, an output for outputting an
amplified optical signal, and a control input for controlling the
gain of the optical amplifier. The cascaded optical amplifier
includes a sensor for sensing upstream of the input of the second
optical amplifier a signal relating to operation of the cascaded
optical amplifier. In addition, the cascaded optical amplifier
includes a controller for providing control signals to the
respective control inputs of the first optical amplifier and the
second optical amplifier, the controller providing the control
signal to the second optical amplifier as a function of the sensed
signal.
[0010] In accordance with another aspect, the sensed signal
represents the optical signal input to the first optical
amplifier.
[0011] According to still another aspect, the sensed signal
represents the amplified optical signal output from the first
optical amplifier.
[0012] According to another aspect, the sensor comprises first and
second sensors, and the sensed signal includes a first sensed
signal representing the optical signal input to the first optical
amplifier and a second sensed signal representing the amplified
optical signal output from the first optical amplifier.
[0013] In accordance with another aspect, the cascaded optical
amplifier further includes at least a third optical amplifier
included in the cascaded arrangement between the first and second
optical amplifiers.
[0014] With still another aspect, the controller provides the
control signal to the second optical amplifier based on a
comparison of the amplified optical signal output or the pump drive
signal from the second optical amplifier and the sensed signal.
[0015] According to yet another aspect, the comparison comprises a
ratio.
[0016] According to still another aspect, the controller includes a
sensed signal delay element for providing a delay to the sensed
signal, the amount of the delay being determined to synchronize
approximately the sensed signal received by the controller with at
least one other signal received by the controller for carrying out
control.
[0017] According to still another aspect, there is little or no
delay between the two amplifiers.
[0018] According to yet another aspect, the cascaded optical
amplifier further includes a delay introducing element coupled
between the output of the first optical amplifier and the input of
the second optical amplifier, and the sensed signal is sensed
upstream of the delay introducing element.
[0019] With still another aspect, the first optical amplifier and
the second optical amplifier are erbium doped fiber amplifiers.
[0020] In accordance with another aspect, the controller delays the
sensed signal as a function of the delay introduced by the delay
introducing element as measured by the controller.
[0021] According to still another aspect, the delay introducing
element is a dispersion compensation module (DCM).
[0022] According to another aspect of the invention, a method of
controlling a cascaded optical amplifier is provided. The cascaded
optical amplifier includes a first optical amplifier and a second
optical amplifier in cascaded arrangement. Each of the first
optical amplifier and the second optical amplifier has a respective
input for receiving an optical signal, an output for outputting an
amplified optical signal, and a control input for controlling the
gain of the optical amplifier. The cascaded optical amplifier
further includes a controller for providing control signals to the
respective control inputs of the first amplifier and the second
amplifier. The method includes the steps of sensing upstream of the
input of the second optical amplifier a signal relating to
operation of the cascaded optical amplifier, and providing the
control signal to the second optical amplifier as a function of the
sensed signal.
[0023] According to another aspect, the sensed signal represents
the optical signal input to the first optical amplifier.
[0024] According to still another aspect, the sensed signal
represents the amplified optical signal output from the first
optical amplifier.
[0025] In accordance with yet another aspect, the sensing step
includes sensing a first sensed signal representing the optical
signal input to the first optical amplifier and sensing a second
sensed signal representing the amplified optical signal output from
the first optical amplifier.
[0026] With yet another aspect, the method includes the step of
providing the control signal to the second optical amplifier based
on a comparison of the amplified optical signal output or the pump
drive signal from the second optical amplifier and the sensed
signal.
[0027] In accordance with another aspect, the comparison includes a
ratio.
[0028] According to still another aspect, the method includes the
step of providing a delay to the sensed signal, the amount of the
delay being determined to synchronize approximately the sensed
signal received by the controller with at least one other signal
received by the controller for carrying out control.
[0029] In yet another aspect, the first optical amplifier and the
second optical amplifier are erbium doped fiber amplifiers.
[0030] According to another aspect, the cascaded amplifier further
comprises a delay introducing element coupled between the output of
the first optical amplifier and the input of the second optical
amplifier, and the sensed signal is sensed upstream of the delay
introducing element.
[0031] According to another aspect, the method includes the step of
delaying the sensed signal as a function of the delay introduced by
the delay introducing element as measured in a measuring step.
[0032] In accordance with yet another aspect, the delay introducing
element is a dispersion compensation module (DCM).
[0033] To the accomplishment of the foregoing and related ends, the
invention, then, comprises the features hereinafter fully described
and particularly pointed out in the claims. The following
description and the annexed drawings set forth in detail certain
illustrative embodiments of the invention. These embodiments are
indicative, however, of but a few of the various ways in which the
principles of the invention may be employed. Other objects,
advantages and novel features of the invention will become apparent
from the following detailed description of the invention when
considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a block diagram of a conventional cascaded optical
amplifier;
[0035] FIG. 2 is a block diagram of a cascaded optical amplifier in
accordance with an exemplary embodiment of the present
invention;
[0036] FIG. 3 is a block diagram representing a simplified control
algorithm for a first amplifier in accordance with an exemplary
embodiment of the present invention;
[0037] FIG. 4 is a block diagram representing a simplified control
algorithm for a second amplifier in accordance with an exemplary
embodiment of the present invention;
[0038] FIG. 5 is an electrical equivalent circuit model of an
add-drop optical signal that is input to the cascaded optical
amplifier in accordance with an exemplary embodiment of the present
invention, an ideal input, and a simulated output;
[0039] FIG. 6 represents a simulated response of a PIN diode used
to sense optical power of the respective amplifiers in accordance
with an exemplary embodiment of the present invention;
[0040] FIG. 7 is an electrical equivalent circuit model of EDFA
included in the cascaded optical amplifier in accordance with an
exemplary embodiment of the present invention, and a simulated
response of the EDFA with respect to an input power change and a
pump power change;
[0041] FIG. 8 graphically represents the response of a DCM as
included in an exemplary embodiment of the present invention;
[0042] FIGS. 9A and 9B represent the simulated performance of a
conventional cascaded optical amplifier of the type shown in FIG.
1; and
[0043] FIG. 10 represents the simulated performance of a cascaded
optical amplifier in accordance with the exemplary embodiment of
the invention as shown in FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
[0044] The present invention will now be described with reference
to the figures, in which like elements are used to refer to like
elements throughout.
[0045] Referring to FIG. 2, a cascaded optical amplifier 40 is
shown in accordance with an exemplary embodiment of the invention.
The amplifier 40 is a two-stage optical amplifier similar to the
conventional amplifier 14 shown in FIG. 1. However, those having
ordinary skill in the art will appreciate that the optical
amplifier 40 could have more than two stages cascaded together
without departing from the scope of the invention.
[0046] The optical amplifier 40 is similar to the conventional
amplifier in FIG. 1 in that the amplifier 40 includes EDFAa and
EDFAb cascaded in series. Again, an add/drop type optical signal is
input to a first EDFAa and its amplified output is input to a
second EDFAb. The output of EDFAa is coupled to the input of EDFAb
via a DCM or other delay causing element that introduces a time
delay to the signal prior to being input to the second EDFAb. The
output of EDFAb represents the output of the cascaded
amplifier.
[0047] Similarly, a first control algorithm 20 provides gain
control of EDFAa. A power coupler or tap 22 senses the power of the
optical signal input to EDFAa and provides a control input PINa1 to
the control algorithm 20. As in the conventional amplifier 14, a
tap 24 senses the power of the amplified optical signal output by
EDFAa and provides a control input PINa2 to the control algorithm
20. The control algorithm 20 compares the output power of EDFAa to
the input power of EDFAa. Based on the desired gain of EDFAa, the
control algorithm 20 provides a gain control signal to EDFAa in the
form of a pump control signal to Pump a. By controlling the laser
pump energy delivered by Pump a, the control algorithm 20 controls
the gain provided by EDFAa.
[0048] The amplified optical signal output from EDFAa is coupled to
the input of EDFAb via a DCM 26. EDFAb in turn further amplifies
the optical signal output from EDFAa. A second control algorithm
42, different from the second control algorithm 30 described above
in connection with the conventional amplifier 14 as explained in
more detail below, serves to provide gain control of EDFAb.
Specifically, a tap 32 outputs a control input PINb1 indicative of
the power of the input signal to EDFAb, and a tap 34 provides a
control input PINb2 indicative of the power of the optical signal
output by EDFAb. The control algorithm 42 receives the control
inputs PINb1 and PINb2 in accordance with one embodiment of the
invention, and compares the input and output signal power of EDFAb
with the desired gain based thereon. The control algorithm 42 in
turn controls the laser pump energy delivered by Pump b based on
such comparison, which in turn controls the gain of EDFAb.
[0049] The optical amplifier 40 of the present invention differs
from the conventional amplifier 14 of FIG. 1 primarily in the
manner in which the control algorithm 42 controls the gain of the
second (or subsequent) amplifier EDFAb among the cascaded
amplifiers. Specifically, one or more control inputs for
controlling the gain of EDFAb are based on signals obtained
upstream of one or more preceding amplifiers in the cascade and/or
the delay element presented by DCM 26 relative to EDFAb. The
present invention utilizes these inputs in order to optimize the
performance of the second (or subsequent) amplifier EDFAb as will
be described in more detail below. For example, the control
algorithm 42 according to the embodiment of FIG. 2 also receives as
inputs control inputs PINa1 and PINa2 representative of the input
and output power, respectively, of EDFAa upstream of the delay
element presented by DCM 26.
[0050] More generally, the present invention relates to using
information from one or more upstream optical amplifiers (e.g.,
EDFAa) included in a cascaded amplifier in order to better optimize
the performance of one or more subsequent optical amplifiers (e.g.,
EDFAb) included in the cascaded amplifier. In the exemplary
embodiment of FIG. 2, the control inputs PINa1 and/or PINa2 are
input to the control algorithm 42. This provides at least two
advantages.
[0051] Firstly, it provides at least one reference input (e.g.,
PINa1) that has not been altered by the preceding amplifier EDFAa.
Thus, any errors or noise presented by EDFAa will not be present in
the reference input provided to the control algorithm 42 for EDFAb.
This avoids errors or noises accumulating with each additional
stage in the amplifier 40.
[0052] Secondly, changes in the add/drop amplifier input signal or
in any previous stage of the amplifier 40 can be used to alter the
response of a subsequent stage even before the amplified input
signal reaches the particular subsequent stage. In a sense, this
gives the subsequent stage a "head start" on any corrections. For
example, the control algorithm (e.g., 42) of a subsequent stage can
use a reference input (e.g., PINa1 and/or PINa2) from a prior stage
(e.g., EDFAa) to control the gain of a subsequent stage (e.g.,
EDFAb). In such case, the reference inputs (e.g., PINa1 and/or
PINa2) are representative of a reference input that has not
undergone a delay due to the delay element DCM 26 and/or inherent
delays of any intervening components. This enables the control
algorithm (e.g., 42) and subsequent stage (e.g., EDFAb) to get a
head start on any corrections relative to the amplified signal
received via the delay element (e.g., DCM 26) or otherwise subject
to delay.
[0053] In an exemplary embodiment described below in relation to
FIGS. 3 and 4, the control algorithm 42 differs from a conventional
algorithm (such as control algorithm 30 in FIG. 1) simply by
substituting control input signal PINb1 with a delayed version of
control input signal PINa1 from the previous stage. The delay
preferably is programmed and corresponds closely to the time delay
between control input signals PINa1 and PINb1. However, the delay
preferably is slightly less than the actual delay. This allows
EDFAb to react even before it receives the amplified optical signal
from DCM 26 on which EDFAb acts upon. The actual delay will be a
function of the parameters of the EDFAs, the responses of the taps
(e.g., 22, 24, 32 and 34), and the responses of any other elements
(e.g., DCM 26) as will be explained in more detail below in
relation to FIGS. 5-8. How much to shorten the programmed delay
relative to the actual delay between PINa1 and PINb1 can be
optimized, for example, by calculation, empirically, measurement of
the time delay between PINa1 and PINb1 upon start up of the
amplifier or in real time, etc., or via any other means.
[0054] Referring now to FIG. 3, a block diagram representing the
control algorithm 20 for EDFAa is shown in simplified form. As will
be appreciated, the control algorithm 20 may be carried out via
primarily hardware, software, or a combination thereof without
departing from the scope of the invention. The control algorithm 20
includes a multiplier 52 receiving an input PINa1. The multiplier
52 also receives as an input a predefined (desired) amplifier gain
setting (e.g., A). The multiplier 52 produces the output of EDFAa
as represented by the product of PINa1 with the desired gain (e.g.,
A)
[0055] The output of the multiplier 52 is input to a subtractor 54
included in the control algorithm 20. The subtractor 54 compares
this output with the pump drive control signal (P) provided to Pump
a for controlling the pump current and thus the gain of EDFAa.
Those having ordinary skill in the art will appreciate that the
pump drive control signal (P) is indicative of the amplified
optical signal output by EDFAa. In particular, the output of an
EDFA tends to approach the value of the pump output, and thus the
pump drive control signal (P) provided to EDFAa at a given time
tends to be indicative of the output of EDFAa. In an actual control
algorithm, the specific value of PINa2 also may be utilized as will
be appreciated by those having ordinary skill in the art.
[0056] The subtractor 54 outputs a difference signal A*PINa1-P
which represents the offset between the control signal P provided
to Pump a and the desired output. Ideally, the output of the
subtractor 54 is zero. The output of the subtractor 54 is input to
an integrator 56 also included in the control algorithm 20. The
integrator 56 integrates the offset so as to output the corrected
pump drive control signal (P) to the Pump a in order to provide the
desired gain (e.g., A).
[0057] The control algorithm 20 in FIG. 3 is conventional and hence
further detail is omitted for sake of brevity. Those having
ordinary skill will appreciate that the control algorithm 20 as
shown in FIG. 3 may also be representative of the control algorithm
30 in the conventional amplifier 14 shown in FIG. 1.
[0058] FIG. 4 illustrates the control algorithm 42 in accordance
with the exemplary embodiment. Similar to the control algorithm in
FIG. 3, again the algorithm 42 is simplified insofar as the current
pump drive control signal provided to EDFAb is taken as indicative
of the output power of EDFAb. As previously described, the primary
difference between the control algorithm 42 and a conventional
control algorithm (e.g., 30 of FIG. 1) is that a delayed control
input PINa1 is substituted in place of control input PINb1 for
controlling the gain of EDFAb. Similar to the control algorithm 20,
the control algorithm 42 may be carried out via primarily hardware,
software, or a combination thereof without departing from the scope
of the invention.
[0059] Specifically, the control algorithm 42 receives the control
input PINa1 from upstream of EDFAb. In this particular example, the
control input from upstream of EDFAb is the input power to the
preceding EDFAa. However, the control input may be derived from any
other signal upstream (e.g., the output power of EDFAa prior to DCM
26) as previously noted. The control algorithm 42 includes a delay
element 58 that receives the control input PINa1. The delay element
58 preferably is adjustable insofar as the amount of time the
control input PINa1 is delayed by the delay element 58. As
previously discussed, the delay preferably corresponds closely to
the time delay between control input signals PINa1 and PINb1. Of
course, in a different embodiment using a different control input
obtained upstream, the time delay provided by delay element 58
would be selected to correspond closely to the relative time delay
between the respective control inputs.
[0060] The control algorithm 42 is otherwise conventional in the
exemplary embodiment. A multiplier 62 outputs a product signal
A'*PINa1 representing the output of the combination EDFAa and
EDFAb. The output of the multiplier 62 is input to a subtractor 64
included in the control algorithm 42. The subtractor 54 compares
this output with the pump drive control signal (P) provided to Pump
b for controlling the pump current and thus the gain of EDFAb. The
subtractor 64 outputs a difference signal A'/(PINa1/PINb2)-P which
represents the offset between the control signal P provided to Pump
b and the desired gain. Ideally, the output of the subtractor 64 is
zero. The output of the subtractor 64 is input to an integrator 66
also included in the control algorithm 42. The integrator 66
integrates the offset so as to output the corrected pump drive
control signal (P) to the Pump b in order to provide the desired
gain (e.g., compound gain A').
[0061] The control algorithm 42 according to the embodiment of FIG.
4 does not utilize the control input PINa2. However, another
embodiment could also use PINa2 to further enhance performance. For
example, the control algorithm 42 may use the control input PINa2
as an indicator of the gain error of EDFAa. In the event EDFAa and
EDFAb are essentially the same type of amplifier, the control
algorithm 42 can assume EDFAb would make the same error. The
control algorithm 42 can then adjust the compound amplifier gain A'
accordingly to compensate for such error. Such an embodiment is
particularly useful in the case where the performance of the
amplifiers EDFAa and EDFAb change in the same way, for example due
to temperature changes or other external influences.
[0062] As previously noted, it is desirable that the control input
obtained upstream in the cascaded amplifier be delayed by an
appropriate amount in order to be used to control the gain of a
subsequent stage. Ideally, the control input should be synchronized
generally with whichever other control inputs are used to control
the gain in the subsequent stage. The particular amount of the
delay will depend on the time delays otherwise avoided as a result
of the control input bypassing one or more elements in the
cascade.
[0063] For example, the embodiment of FIGS. 2 and 4 substitutes
control input PINa1 for PINb1. Unlike PINb1, control input PINa1 is
not subject to the time delays associated with EDFAa, tap 24, DCM
26 and tap 32. Thus, the delay amount provided by delay 58 should
be adjusted so as to be approximately equal to the combined delay
of EDFAa, tap 24, DCM 26 and tap 32. Further, the delay 58 may take
into account delays associated with the add/drop optical input
signal itself, as discussed below in relation to FIG. 5. It will be
appreciated, however, that the particular delay depends primarily
on the particular signals being utilized by the control algorithm,
the particular configuration of the cascaded amplifier, etc.
[0064] As previously noted, the particular time delay provided by
delay 58 can be optimized, for example, by calculation,
empirically, measurement of the time delay upon start up of the
amplifier or in real time, etc.
[0065] Referring to FIG. 5, the response time of the add/drop
signal input to the amplifier 40 may be calculated. FIG. 5
illustrates how the add/drop input signal may be modeled as a
single low pass filter 70 having a resistor 72 and capacitor 74.
The input to the filter 70 is an ideal optical step function as
shown in graph 76 representing the drop and subsequent addition of
an optical input. The output of the filter 70 as represented in
graph 77 illustrates the delay introduced by the filter via the
time constant associated with the filter 70. In the exemplary model
shown in FIG. 5, the add/drop signal input exhibits a delay of
approximately 5 microseconds (.mu.s).
[0066] FIG. 6 represents the response of each of taps 24 and 32.
The taps 24 and 32 also may be modeled as single pole low pass
filters. In the exemplary embodiment, the time constant of the taps
24 and 32 results in a delay of approximately 0.1 .mu.s.
[0067] FIG. 7 represents a model 78 of an EDFA such as EDFAa and
EDFAb. The model includes a time delay 80 that delays the optical
input by a fixed time. The output of the time delay 80 is coupled
via a capacitor 82 to output node 84. The pump input is modeled as
a resistor 86 coupled to the output node 84. The response of the
EDFA in relation to a change in optical input power is shown in
graph 88. The response of the EDFA in relation to a change in pump
power is shown in graph 90. As is noted in each case, there is an
overall time delay associated with the delay 80 and RC component
provided by capacitor 82 and resistor 86. In the exemplary
embodiment, the time delay due to delay 80 is approximately 0.2
.mu.s and the time delay due to the RC component is approximately
20 .mu.s.
[0068] FIG. 8 represents a model of the DCM 26. The DCM 26 may be
modeled simply as a time delay element 92. As is shown in graph 94,
the output of the DCM 26 is a simple delay of the input. In the
exemplary embodiment, the time delay is approximately 140 .mu.s.
Thus, in an amplifier such as that shown in FIG. 2 in accordance
with the present invention, the DCM 26 represents the predominant
delay between PINa1 and PINa2 relative to PINb1 and PINb2 as will
be appreciated.
[0069] Further, although not shown in the Figures, the control
algorithm 42 may itself have a delay associated with the processing
time to carry out the appropriate control functions. For example,
the control algorithm may have a delay due to processing of
approximately 1 .mu.s.
[0070] Generally speaking, the delay between PINa1 and PINa2 is
simply the EDFA transition time that not only is quite short, but
is also known and largely unchanging. The same may be said with
respect to the delay between PINb1 and PINb2. The delay between
PINa1 and PINb1 may not always be known. For example, the DCM 26
may be configured in the field (i.e., at the time of installation)
rather than at the time of production of the amplifier. In such
case, the delay between PINa1 and PINb1 can be measured at startup
following installation in the field.
[0071] For example, the PINa1 to PINb1 delay can be determined upon
startup by having the overall control algorithm for the amplifier
40 modulate the ASE noise of the first EDFAa by modulating its pump
intensity. This would be detected in either of PINb1 or PINb2. A
cross correlation between the modulated pump signal and the
detected PINb1 or PINb2 can be used to determine the approximate
delay presented by the DCM 26. The control algorithm 42 may then
configure the delay 58 to provide such delay.
[0072] Once the delay for delay 58 is initially determined, the
PINa1 to PNIb1 delay can be optimized and tracked by periodic
measurements of the cross correlation function between PINa1 and
PINb1 (or any of the PINa signals with any of the PINb signals) any
time the input signal changes. This can be done as an overhead
calculation as the changes are expected to be slow (usually caused
by thermal variations). Indeed, a modulation of the EDFAa pump
intensity can also be used while the amplifier is active as it can
be canceled out using the subsequent EDFAb.
[0073] FIGS. 9A and 9B illustrate the deviation from ideal of a
conventional amplifier 14 such as that shown in FIG. 1. The various
time delays associated with the different components in the
conventional amplifier are assumed to be equal to the corresponding
components in the amplifier of FIG. 2 in accordance with the
present invention. FIG. 9A illustrates a change in the power of the
add/drop signal and the related changes in the outputs of EDFAa,
DCM 26 and EDFAb. As noted above, the time delay of the DCM 26 and
the time delays due to the response times of the EDFAs are
substantially greater than the delays introduced by the other
components. Consequently, the power of the add/drop signal and the
output of EDFAa closely follow one another according to the scale
of FIG. 9A and are represented collectively by composite line 96.
Likewise, the output of DCM 26 and the output of EDFAb closely
follow one another and are represented collectively as composite
line 98.
[0074] As previously explained, the delay due to DCM 26 is
intentionally provided within the cascaded amplifier. FIG. 9B,
taking into account the intended delay of DCM 26, represents the
deviation of the outputs of EDFAa, DCM 26 and EDFAb in relation to
their ideal outputs according to the conventional amplifier. The
dashed line in FIG. 9B represents the deviation of EDFAa relative
to its ideal output. As is noted, the deviation of EDFAa peaks at
approximately 2.0 decibels (db). This same deviation is carried
thru to the DCM 26 whose output also then deviates by approximately
2.0 db as represented by the dotted line.
[0075] The solid line in FIG. 9B illustrates the deviation in the
output of EDFAb from its ideal output. Due to the deviation in the
output from EDFAa in combination with the deviation introduced by
EDFAb itself, the output of EDFAb deviates from its ideal at a peak
of approximately 3.5 db as shown in FIG. 9B.
[0076] In comparison, FIG. 10 illustrates the response of the
amplifier 40 of FIG. 2 in accordance with the exemplary embodiment
of the present invention. The add/drop power input and associated
component delays are identical to that represented in FIG. 9A with
respect to the conventional amplifier 14. According to the
exemplary embodiment of the invention, however, the control input
PINa1 is substituted for PINb1 in the control algorithm 42.
Consequently, any errors otherwise introduced by preceding EDFAa
are effectively bypassed and thus are not input to EDFAb. The delay
element 58 is configured to provide a time delay of 161.2 .mu.s
based on the above-discussed models of the respective components
(e.g., EDFA=20.2 .mu.s, DCM=140 .mu.s,).
[0077] As is shown in FIG. 10, the deviation in the outputs of
EDFAa and DCM 26 remain unchanged. However, the deviation in the
output of EDFAb from its ideal is substantially less than in the
case of the conventional amplifier 14. More specifically, the solid
line in FIG. 10 illustrates how the output of EDFAb has a peak
deviation of 1.2 db. This represents an improvement over the
conventional amplifier of over 2 db.
[0078] Thus, it will be appreciated that the present invention
provides a significant improvement in the performance of the
amplifier.
[0079] Although the invention has been shown and described with
respect to certain preferred embodiments, it is obvious that
equivalents and modifications will occur to others skilled in the
art upon the reading and understanding of the specification. For
example, the cascade amplifier 40 in accordance with the present
invention may include more than two EDFAs cascaded in series
without departing from the scope of the invention The invention has
application with respect to any subsequent amplifier stage
receiving as a control input a signal obtained from a preceding
stage. The preceding stage need not be the immediately preceding
stage as will be appreciated. Rather, the preceding stage can be
any preceding stage.
[0080] Further, the basic control algorithms described herein can
be further revised to optimize the response in accordance with the
present invention. For example, the impulse response of the EDFAs
(e.g., to the input signal and/or the pump intensity) can be
determined to the extent they are linear. The impulse response can
be used to calculate an inverse to the input signal measured before
any delay (e.g., DCM 26). The inverse response is then provided to
the pump of the subsequent EDFA to optimize its response.
[0081] While the present invention has been described herein as
having separate control algorithms for the respective EDFAs, it
will be appreciated that each of the particular algorithms may be
referred to collectively as part of the same controller.
[0082] The present invention includes all such equivalents and
modifications, and is limited only by the scope of the following
claims.
* * * * *